1. Field of the Invention
The present invention relates to a variable optical element that is capable of, for example, altering optical characteristics such as diffraction efficiency, a pickup apparatus that comprises the variable optical element, and an information recording and reproducing apparatus that comprises the pickup apparatus to carry out the recording of information to and reproduction from information recording media.
2. Description of the Related Art
In recent years, information recording media having recordable properties other than information recording media having reproduction-only properties and of a phase-change type have received attention as high-density and large-capacity information recording media. Among the phase-change type information recording media, write-once optical discs that allow one-time recording of information and rewritable optical discs that allow erasing and rewriting of information have been received attention.
These phase-change type information recording media have a layered structure, as shown in the sectional view of, for example, comprising a light-incident layer onto which a light beam is made incident from an objective lens provided in a so-called pickup apparatus, a recording layer of a crystal or amorphous material protected by a protection layer, a reflective layer, and a substrate, and are so configured that information is recorded by effecting phase changes to the recording layer by the light energy of a light beam. Reproduction of information is carried out by first irradiating a light beam onto the recording layer, which reflects the beam, then converging rays of reflected light with the objective lens, and finally detecting the reflected light with a photodetector of the pickup apparatus.
Write-once optical discs have a characteristic whereby they are capable of recording information only once, which is provided for by having a recording layer whose phase is changed (an irreversible change) according to light energy. Erasable optical discs have a characteristic whereby they are capable of rewriting information, which is provided for by having a recording layer whose phase is changed (a reversible change) according to light energy. In addition, the recording density of these information recording media are increased by narrowing the track pitch of the recording layer and further increases in recording capacity are rendered by adding more recording layers.
It is considered, on the other hand, that a light beam of a smaller focused spot diameter is irradiated onto a recording layer by using a greater numerical aperture (NA) of an objective lens provided in a pickup apparatus in order to realize an information recording/rewriting apparatus that keeps pace with the above mentioned high-density information recording media.
However, while increasing the numerical aperture (NA) of the objective lens corresponding to narrower track pitches of information recording media, it is important to carry out high-density information recording and appropriate information reproduction within the limited range of the numerical aperture (NA) of the objective lens. As a response to this problem, attention has been given to the usefulness of applying a crosstalk cancellation technique when reproducing information.
A crosstalk cancellation technique is such that not only a light beam is (referred to as the main beam) irradiated onto a track in which information is recorded (referred to as the main track) to read the information from it but also light beams (referred to as the sub-beams) are irradiated onto the tracks next to the main track (referred to as the adjacent tracks) to read information from them as well, whereupon by executing a predetermined calculation based upon the main signal obtained from the main beam and the sub-signals obtained from the sub-beams, a crosstalk component contained in the main signal is controlled to reproduce information with a good accuracy.
In addition, a method is considered to diffract a laser beam emitted from a laser light source with the use of a diffraction grating in order to generate the above mentioned main beam and sub-beams, and thereby irradiate the 0th order light as the main beam and +/− 1st order light as the sub-beams onto each corresponding track through an objective lens.
However, while above mentioned crosstalk canceling technique is applied to the reproduction of information from a reproduction-only information recording medium such as a DVD-ROM (digital versatile disc read only memory), which proves effective in reproducing information with good accuracy, if it is applied to the reproduction of information recorded on a phase-change type information recording medium such as a DVD-RW(digital versatile disc rewritable), it poses problems as described below.
In the case of a phase-change type information recording medium, the phase of the recording layer alters corresponding to the amount of light energy exerted. For this reason a problem is caused where, when reproducing information from a phase-change type information recording medium, irradiating the above mentioned main beam and sub-beams used for a reproduction-only information recording medium against the corresponding main track and sub-tracks of the phase-change type information recording medium imparts phase changes to the recording layer of the main track and/or sub-tracks to erase or destroy information that was recorded. In other words, a problem occurs where, if the same light beam is applied to both a reproduction-only information recording medium and a phase-change type information recording medium, it is suitable for one type of information recording medium but not suitable for the other type of information recording medium.
To avoid such a problem, a method can be considered to adjust the light energy levels of the main beam and sub-beams, by replacing the diffraction grating with another having a different diffraction efficiency for each particular case, depending on whether information is recorded to a phase-change type information recording medium, or information is reproduced from a phase-change type information recording medium, or where information is reproduced from a reproduction-only information recording medium which is not of a phase-change type. However, this method will give way to a problem in which a rather large-scale mechanism is required to replace one diffraction grating with another.
In addition, replacing the grating causes another problem where optical characteristics of the optical system including the above mentioned objective lens may become unstable.
Further, since replacement of the diffraction grating will take some time, when, for instance, recording and reproducing information to and from a phase-change type information recording medium is repeated one after the other, it will give rise to the problem of responsiveness in that recording and reproduction of information may not be carried out rapidly.